1. Field of the Invention
The present invention is directed to: nanomaterials; to nanotubes; to nanoribbons; to metallized nanomaterials; in one particular aspect, to metallized carbon nanotubes; and to methods for metallizing nanomaterials, e.g. nanotubes, graphene sheets, and graphene ribbon-like material.
2. Description of Related Art
A wide variety of nanomaterials are known, e.g. nanotubes and nanoribbons. Carbon nanotubes (CNTs) have attracted much attention because of their extraordinary mechanical properties and unique electronic properties. A CNT is topologically equivalent to a two-dimensional graphene sheet rolled into a cylinder, with a cylinder diameter as small as 0.7 nanometers (nm) (and as large as several tens of nanometers) and with a cylinder length up to several microns (μm). A CNT can be single walled (SWNT) or multiple walled (MWNT) and can also be fabricated as a fiber or other structure. A CNT can be characterized by its chiral vector components (n, m), which help define tube diameter, electronic properties and other properties. Depending upon the chirality, a SWNT can be conducting (metal-like) or semiconducting.
Currently, attempts are ongoing to utilize CNT's to enhance the properties of a variety of polymers and composites. Significant mechanical, electrical, and thermal property improvements have been reported. However, in certain cases, there are significant processing obstacles that prevent full enhancement of polymers and composites using incorporated nanotubes. In certain methods, dispersion has been shown to be essential for property enhancement when nanotubes are blended with polymers. Due to the intrinsic van der Waals attraction the nanotubes have to each other, and by virtue of their high aspect ratio (e.g., about 1:1000), nanotubes are typically agglomerated along their lengths, e.g. together as bundles and ropes, that have very low solubility in most solvents. In many instances, despite processing to achieve individual particles, nanotubes tend to remain as entangled agglomerates and homogeneous dispersion is not easily obtained. Furthermore, due to the atomically smooth non-reactive surface of nanotubes, lack of interfacial bonding limits load transfer from the matrix to nanotubes. In this situation, nanotubes are often pulled from a matrix, rather than fractured, and play a limited role in mechanical reinforcement of a composite structure.
The potential for certain uses of nanotubes in these applications has, heretofore, not been realized because of difficulties in processing and limited CNT-to-matrix load transfer. Existing methods for overcoming these difficulties include chemical functionalization (addition of one or more specified chemical groups to a basic structure) and metallization (deposition of metal nanoparticles) on the CNT's. Development of enhanced polymers and composites may require functionalization of a collection of CNTs to allow the tubes to be dispersed more easily in a host polymer.
Certain current known CNT chemical functionalization processes require complex apparatus and processing, use wet chemical procedures, and involve liquids or vapors, to which the CNTs are exposed. An example is the use of hot flowing fluorine gas to bond fluorine atoms to CNTs, as reported by E. T. Michelson et al in Chem. Phys. Lett. vol 296 (1998) 188. Large quantities of chemical reactants and solvents are often required, with most of the chemicals becoming residues that must be disposed under hazardous substance guidelines. Recycling of the chemicals used is seldom an option.
Current processes for functionalization of CNT's by metallization include a physi-sorption technique that involves the separate preparation of metal nanoparticles involving a long sonication process to mix them with the carbon nanotubes, where dislodging of the metal particulates is often observed due to their loose attachment. Electroless deposition requires a harsh oxidative pretreatment (of the CNT's) followed by a complex activation-sensitization procedure, which is disruptive to the intrinsic CNT structure and properties.
Electrochemical deposition is challenging because of the need to establish reliable electrical contact with bulk CNT substrates. Current approaches to overcoming this challenge include CNT growth on a conducting template, microlithography, electrophoresis, sputtering and thermal evaporation, all of which involve complex apparatus and processing and are not amenable to scale-up for industrial manufacturing.
Known methods for metallizing nanotubes include, but are not limited to, those disclosed in U.S. patent applications Ser. No. 11/021,129 filed Dec. 22, 2004; Ser. No. 10/372,006 filed Feb. 21, 2003; Ser. No. 10/813,697 filed Mar. 31, 2004 and Ser. No. 12/232,818 filed Sep. 24, 2008; and in PCT Application Int'l Publication No. WO 2008/140623A1 published Nov. 20, 2008 (all said applications incorporated fully herein for all purposes).
There is a need for improved nanotube metallization processes that do not require complex apparatus or processes, produce relatively little residue for disposal, are efficient, selective, reasonably fast, and are scalable to large industrial scale production. The present invention recognizes these needs and provides solutions to these problems.